1. Field of the Invention
This invention relates to an electrolytic process for forming a hard protective film on a metallic surface and to products made by the process.
2. Description of the Prior Art
Although metal surfaces which are painted or enameled can resist chemical attack, such organic coatings have other disadvantages such as degradation at high temperatures. Frequently, paints, enamels and the like do not adhere well to metallic surfaces, especially when temperature cycling is involved.
A well known electrolytic process for providing a protective coating on metals is the anodization of aluminum. Various other chemical and electrochemical processes have been used in attempts to provide a durable protective coating for metals and their alloys.
U.S. Pat. No. 1,923,539 relates to the formation of protective coatings on aluminum, magnesium and their alloys by using alternating current at about 100 volts. Various solutions are mentioned as suitable for the coating process. U.S. Pat. No. 2,380,044 concerns the coating of metal articles by electrodeposition using molybdenum compounds. The baths discussed in this patent always contain another metal, preferably nickel, in addition to molybdenum.
However, none of the prior art methods had provided a coating that is highly resistant to attack by corrosive agents such as strong acids and strong alkalis. Anodized aluminum for example, is not resistant to either acids or mild alkalis.
Other methods have been proposed for the coating of metal and alloy surfaces for special purposes. For example, U.S. Pat. No. 3,625,737 relates to the provision of a silicate coating on aluminum reflector surfaces by means of a number of treatment steps intended to avoid "hazing".
Czechoslovakian Patent No. 104,927 to Hradcovsky and Belohradsky relates to the production of a bypass element for electrical circuits by coating an aluminum substrate electrolytically with a silicate layer in a weakly alkaline aqueous bath comprising either sodium or potassium silicate and a small amount of ammonium molybdate.
U.S. Pat. No. 1,748,012 shows a dry surface rectifier cell using aluminum plates coated in a borax solution, and suggests that other metals could be similarly treated to produce rectifier plates or washers. U.S. Pat. No. 1,827,204 shows the coating of iron, zinc, cadmium, copper and silver by electrolytic treatment with a chromium-containing bath.
U.S. Pat. No. 1,925,307 to DeBoer et al. shows the formation of a layer of an oxide on zirconium, titanium, thorium and hafnium for use in electrical condensers. U.S. Pat. No. 2,019,546 shows the production of oxide cathodes with coatings of alkaline earth metal bronze. U.S. Pat. No. 2,231,373 relates to the coating of aluminum by means of electrolysis in a bath containing titanium salts.
U.S. Pat. No. 2,380,044 shows the deposit of a molybdenum compound on aluminum by electrodeposition from a bath which may contain sodium hydroxide. U.S. Pat. No. 2,417,133 also shows the deposition of molybdenum-oxygen deposits on aluminum.
U.S. Pat. No. 3,305,384 shows the production of aluminum coated iron surfaces, and U.S. Pat. No. 3,457,145 relates to the preparation of metal film resistors and shows the anodization of aluminum coated strips.